Leading sports doctor Peter Larkins said the two substances had an opposite effect.

''One puts you to sleep, one keeps you awake,'' Larkins told Fairfax Media. ''Phentermine is part of the go-fast group, as I call them, which is similar category to all of the adrenalin amphetamines … anything that is a stimulant that keeps you awake and improves your reflexes."

Advertisement

According to witnesses who saw Botha in his dressing room before the fight, he was extremely relaxed and was lying on the floor as Williams entered the ring first.

However, once the fight began he was alert and continually taunted Williams, while trying to put him off his game with a series of late shots after the bell to end each round or when the referee had separated the pair.

After Williams survived a frenzied 10th-round attack by Botha, who got stronger as the fight continued in the 10th round, to win a unanimous points decision, the South African claimed he had expected a 12-round bout.

His promoter Thinus Strydom said afterwards that he had known shortly before the bout that it would be fought over 10 rounds but did not tell Botha.

However, Botha's coach, Hardy Mileham, said at a media conference in Durban late last year that they were preparing for a 10-round bout.

"We are training for a 10-round fight and we do not want to undermine him,'' Mileham said. "We are training as if we are fighting for the world heavyweight championship.''

According to website Boxrec, the previous four bouts for the WBA international heavyweight title were also sanctioned as 10-round fights.

Williams, who dominated his sixth fight for the opening eight rounds against an opponent having his 61st bout, had been prepared to consider a rematch with Botha in Cape Town.

However, a potential two-year ban for Botha is likely to end the career of a boxer who had traded blows with the likes of Mike Tyson, Lennox Lewis, Evander Holyfield and Wladimir Klitschko.

Williams will turn his attention to the NRL and will train for the first time with Sydney Roosters on Monday.